Residents of a tiny English village have been kept awake for the past two months by a persistent low hum, which arrives every night at midnight and goes until 4 a.m., and nobody knows what causes it.

People in Woodland, County Durham, variously describe the sound as a throbbing, a buzzing, and a droning – all occurring just at the edge of perception. Many have likened it to the sound of an idling diesel engine.

Not everyone can hear it. The BBC reports that most of the complainants are between 50 and 60 years old.

The Durham Hum, as it has become known, is not the world's first unexplained low frequency rumble. In the 1970s, thousands of people in Bristol, England, complained of a deep throbbing sound. Nobody ever discovered the the source, and one day the Bristol Hum simply stopped.

What's more, those attempting to record the resonating thrum usually end up with sound tracks that are completely silent.

Many have sought to explain the source of these maddening drones. Potential culprits have ranged from the conventional – such as abandoned mine shafts, colliding ocean waves, automobile traffic, airplanes, and factory equipment – to the crackpot. A 2009 story in Britain's Daily Mail reports that many hearers have attributed The Hum, as it is known, to secret government experiments, satellites, and even UFOs.

The phenomenon inspired an episode in Season 6 of the "X-Files," in which a secret government experiment in extremely low-frequency radio transmissions caused people's heads to explode.

But some experts say that the truth may not be out there, as such, but originating in the heads of those hearing it. One man who has studied this phenomena for years, Dr. David Baguley, director of audiology at the Addenbrooke's Hospital in Cambridge, has concluded that the hum is created when people fixate on innocuous background noises and train their hearing to become sensitized to it, even continuing to "hear" it after the original noise is gone.

"It becomes a vicious cycle," Baguley told the BBC. "The more people focus on the noise, the more anxious and fearful they get, the more the body responds by amplifying the sound, and that causes even more upset and distress."

But Woodland residents insist that the hum is very real.

"If I put my fingers in my ears it stops," Marylin Grech told the Telegraph. "So I know it's not in my head."